1. Field of the Invention
The field of the invention relates to computed tomography (“CT”)-based threat detection systems generally, and more particularly to certain new and useful advances in using X-ray Diffraction (“XRD”) to resolve false alarms generated by a megavoltage CT threat detection system, of which the following is a specification, reference being had to the drawings accompanying and forming a part of the same.
2. Discussion of Related Art
Megavoltage CT is used for inspection and screening of shipping containers, as it provides a high-resolution, 3-D data set of the density and approximate atomic number distribution inside the container. That said, Megavoltage CT is not material-specific. Rather it can generate a significant number of false alarms for some cargo categories. Some of these alarms can be cleared employing On-Screen Alarm (“OSAR”) protocols. Others, however, need to be cleared by a secondary technique, or, in a worst-case scenario, the containers that generate an alarm must be manually inspected. It is desirable to avoid employing a secondary technique or manual inspection because both operations are extremely costly.
XRD is a material-specific analysis technique that permits the local diffraction properties of a selected volume element (voxel) of an extended object (e.g. a suitcase, a piece of passenger baggage, a shipping container, and the like) to be determined. The idea of operating an XRD system as a second inspection modality that follows a first CT investigation to reduce the false alarm rate in airport baggage screening has been generically outlined in the scientific literature. But the mere mention of this idea left many problems unsolved.
One such problem is whether and how materials such as, for example, high density metals (“HDMS”) and shielded special nuclear materials (“SNMs”), that are present in the extended objects are detected with a CT-based threat detection system in combination with an XRD-based threat detection system. Another problem is how to optimize an XRD threat detection system to minimize its total X-ray attenuation. Still another problem is how to permit simultaneous XRD imaging from a one-dimensional array of object voxels.
Accordingly, an improved threat detection system is needed that detects HDMS and shielded SNMs, that minimizes or eliminates false alarms, and that determines a minimum attenuation path through an extended object. An improved secondary collimator is also needed that enables simultaneous XRD imaging from a one-dimensional array of object voxels.